In exhaust gas discharged from a cogeneration plant, a coal-fire thermal power plant, a heavy-oil plant, and the like, generally, harmful substances such as hydrogen chloride, sulfur oxides, NOx and dioxins are contained in large amounts. As a method of specifically removing NOx among the harmful substances, a selective catalytic reduction method (SCR) with excellent removal efficiency, selectivity, and economy has been widely applied. In an SCR process, for removing NOR, a reductant such as NH3, urea, hydrocarbon and the like is used and NOx is reduced to harmless gas such as N2 or H2O by using the reductant on the catalyst. In the SCR process, a V2O5—WO3/TiO2 catalyst has been widely used and is prepared by mixing additives such as a small amount of V2O5 as a main active component of the catalyst, WO3 for increasing stabilization and durability of the catalytic activity, and SiO2 for mechanical stability of the catalyst while using TiO2 as a carrier.
Meanwhile, even in most of NOx emission facilities such as a cogeneration plant, a coal-fire thermal power plant, and a heavy-oil plant in the domestic contrary as well as advanced countries, the SCR process for reducing NOx is already installed or being installed. The lifespan of the catalyst used in the SCR process of using ammonia as a reductant slightly varies depending on an environment in which the catalyst is operated at respective workplaces, but is approximately 3 to 5 years. Only a few years ago, expired waste denitrification catalyst with significantly deteriorated activity was classified as a specific waste and was embedded. However, considering the economic feasibility of the valuable metals included in the waste denitrification catalyst, a roasting with soda process or an alkali no-pressure leaching process was developed and a method of recycling the waste catalyst as a method of recovering the valuable metals included in the waste denitrification catalyst is being developed.
However, a method of recovering vanadium and tungsten which are leached in the roasting with soda process or the alkali no-pressure leaching process for recovering the valuable metals is required, and a method capable of efficiently recovering vanadium and tungsten with a high recovery rate is required.
As a prior art related with this, Korean Patent Application Publication No. 10-2003-0089401 (published on Nov. 21, 2003) discloses a method for separating and recovering vanadium, tungsten, and titanium from a waste denitrification catalyst.